We have developed a new classification of human pituitary adenomas based on electron microscopy and immunocytochemistry. We can now recognize the following distinct entities: (1) growth hormone cell adenomas; (2) prolactin cell adenomas; (3) mixed adenomas consisting of growth hormone cells and prolactin cells; (4) corticotroph adenomas; (5) thyrotroph adenomas, and (6) undifferentiated cell adenomas. The first two types can be further subdivided into densely-granulated and sparsely-granulated forms. The above classification is proposed as a framework upon which to build additional experience that will lead to its modification. The most important novel contribution of the work to date is the concept that pituitary hypersecretion syndromes (such as acromegaly or galactorrhoea-amenorrhoea due to prolactin excess) may have several different pituitary lesions that can be recognized with appropriate study. For example acromegaly may be caused by one of three different types of pituitary tumor, either densely-granulated somatotroph adenoma, or sparsely-granulated somatotroph adenoma, or mixed somatotroph and lactotroph adenoma. To study structure-function relationships, preoperative pituitary investigation will include, if possible, when appropriate, inhibition tests using bromergocryptine and cyproheptadine. In the past four years this work has led to the recognition of three new human pituitary tumor types, namely oncocytoma, "silent" corticotroph adenoma and acidophil stem cell adenoma (composed of immature cells with combined features of somatotroph and lactotrophs). From the examination of many more tumors additional new entities will likely be defined. We wish to establish an international pituitary tumor reference service that would generate valuable new information about this subject.